The present invention relates to apparatus for processing the output signals from CCDs (charge coupled devices), such as used for imaging and more particularly, to such processing that improves the S/N (signal-to-noise ratio).
When using a CCD imager, the output signal is derived from a clocked "C" register. This output signal can be derived from either an on-chip charge-to-voltage converter, such as a floating diffusion, or the signal current may be obtained directly from a drain diffusion, such as a reset drain. If the output signal is derived from the floating diffusion, it is buffered by a MOSFET amplifier which must be integrated onto the same chip as that in which the imager is formed to achieve low input capacitance. However, such transistors have a large 1/f noise, i.e., the noise energy per unit of bandwidth increases as frequency decreases. In particular, this noise is especially troublesome from D.C. to about 100 kilohertz and causes random horizontal streaks and flicker in the displayed image. These streaks and flicker are highly visible under low illumination conditions. If the output signal is derived from the reset drain, it is applied to an amplifier that can be made to have low 1/f noise by using a J-FET (junction-field-effect transistor), which inherently has high capacitive input loading. This loading causes a loss of high frequencies, which loss may be compensated by high frequency peaking. This peaking causes increased high frequency noise, which is displayed in the reproduced picture as "snow" or "salt and pepper noise".
It is therefore desirable to process output signals from a CCD to maximize the signal-to-noise ratio to thereby minimize streaking and snow in the reproduced picture.